The M80's have great bass for 'only' having 6.5 inch drivers, but there is no substitute for a true subwoofer. Make no mistake, they sound excellent on their own, and I will switch back and forth between solo M80's and M80 + Sub on my setup, depending on what I'm listening to.
I would say that a lot depends on what kind of music you listen to. Classical and Jazz sound lovely when produced by the M80's on their own. On a good classical recording, the M80's just sing and put the orchestra in your living room. Pop & rock stuff sound better with the sub to kick out the bass lines. Things with heavy bass (think celtic drums, techno bass lines, etc) definitely benefit from a dedicated sub.
All told, I usually run my setup with both the M80's and my sub, with a 50 hz crossover.
